A particular object of the invention is a reflectometric method of measurement and a corresponding apparatus where the sample to be measured is diffusely illuminated. In the prior art such a method and apparatus are called Elrepho (short for Electric Reflectance Photometer). Reflectometric methods of measurement are employed while measuring the whiteness, fluorescence, brightness, transparency, opacity and clarity of the samples, or other such reflectometric optical properties. An even more specified object of the invention is a reflectometric method of measurement and apparatus for measuring the ISO (International Standard Organisation) whiteness in paper, pulp and paper-board, i.e. in various paper products.
Nowadays paper whiteness is practically everywhere measured almost exclusively by using the two-channel reflectometric measuring apparatus manufactured by Zeiss. The apparatus comprises a ball-like measuring chamber which is white inside and is provided with two channels, i.e. microscopes complete with diaphragms, and a sample aperture and a standard sample aperture, placed respectively in line with the channels, the measuring channel and the reference channel. The light intensities in the channels are detected by means of photometers located in the channels, and the light intensities in the channels are observed by means of a galvanometer, whereto the photometers are connected. The apparatus is calibrated so that the standard sample with a known reflectance value is placed in the reference channel, and the measuring diaphragm of the reference channel is set to correspond to the known reflectance value of the standard sample. Thereafter the diaphragm of the measuring channel is adjusted so that the light intensities are equally strong, i.e. in equilibrium in both channels. The reflectance value of the sample to be measured is defined by placing the sample in the measuring channel and by adjusting the measuring diaphragm in such a position that the two channels are in balance; now the reflectance value of the sample can be read on the scale of the measuring diaphragm. During the measurement of the sample, the diaphragm of the measuring channel is kept in the position adjusted in the calibration measurement.
The above described prior art method and apparatus for reflectometric measurements have, however, proved to be unsatisfactory as the demands for the accuracy of the measurements have grown. Particularly the adjusting of the diaphragm in the measuring channel--whereby the channels are set in balance and the scale of the measuring diaphragm in the reference channel is defined in connection with the calibration--is inaccurate; in other words, the accuracy of the adjustment is dependent on the operator and his personal accuracy. Moreover, the described adjusting of the diaphragm in the measuring channel must be carried out anew for almost every measurement.
Furthermore, the scale of the measuring diaphragm of the reference channel is based on the scale of measurement engraved in the measuring diaphragm while the device has been manufactured, which scale is only set to a desired level during the calibration by aid of a standard sample. Consequently, for instance when the measuring equipment becomes aged, the deviation between measuring results from separate apparatuses is increased, and the measuring results thus become unreliable.
Moreover, the above described method and apparatus of measurement are based on the presupposition that the reflectance values of the samples are linearly dependent on the position of the measuring diaphragm, i.e. that the scale of the measuring diaphragm is made linear. However, thorough measurements have shown that the reflectance values of different samples do not change in a linear fashion along with the adjusting of the dimming. Consequently, erroneous results in measurement are inherent already in the structure of the method and apparatus of the prior art.
Owing to the above explained and other reasons, the reflectometric methods and apparatuses currently in use have proved out to be largely unsatisfactory. Thus for instance reflectance measurements of paper, carried out both by the manufacturer and the purchaser, have given considerably differing results--in other words, the measuring methods and apparatuses do not render reproducible, comparable results. In practice the lack of reproducible and comparable results in reflectance measurements has for instance lead to situations where paper batches have been returned to the mill, although both the buyer and the dealer have in turn checked that the particular batch fulfils the requirements as regards the reflectance measurements.